The present invention relates to apparatus and method for diagnosing an occurrence of misfire in any one or more of combustion chambers of a plurality of engine cylinders of an internal combustion engine on a basis of a variation in an angular velocity of the engine revolutions.
A Japanese Utility Model Registration Application First Publication No. Heisei 5-17172 exemplifies a previously proposed misfire diagnosing apparatus in which a period of a reference signal REF outputs from a crank angle sensor is measured, a variation in an angular speed of the engine is determined on the basis of the measured period so as to determine a presence or absence in the misfire.
The period at which the reference signal REF is generated is needed to correspond to a constant crank angle (crankshaft rotation angle). However, in an actual practice, it is often the case that the reference signal REF cannot be generated for each of the constant crank angles due to a mechanical deviation such as a crank angle sensor positional deviation. For example, although the generation of the reference signal REF is set at the period of 180.degree. CA (crankshaft rotation), the reference signal REF is actually generated at a smaller or larger crank angular interval than the normal 180.degree. CA. Therefore, an accuracy of the misfire-diagnosis becomes lower due to the variation in the period of the generation of the reference signal REF caused by the variation in the crank angle.
It is necessary to correct the period of generation of the reference signal REF so as to cope with the mechanical deviation caused by the sensor positional deviation in order to improve the accuracy of the misfire diagnosis. However, during the engine driving condition, the period described above is varied not only due to the mechanical deviation but also due to combustion deviations between respective cylinders. In addition, the variations in the period described above due to the mechanical deviation and due to the combustion deviations cannot be distinguished. Hence, since a characteristic of the deviation in the period described above due to the mechanical deviation cannot be determined, the correction of the period of the reference signal REF cannot accurately be carried out against the mechanical deviation.